The present invention relates to a circuit for detecting a ringing which arrives over a telephone subscriber's line in order to identify reception of a call.
As well known in the art of telephones, a ringing, or calling signal, is implemented with an AC signal which intermittently appears over each predetermined duration and disappears over each predetermined duration. A network control device is adapted to automatically respond to reception of a call at a facsimile apparatus or like subscriber's apparatus which may be connected to a subscriber's line. A predominant type of network control device is constructed to decide that a call has been received by detecting the ringing. One of prior art ringing detection circuits includes a full-wave rectifier which is implemented with a diode bridge, and a threshold circuit which is implemented with a Zener diode, a photocoupler, etc. In such a prior art circuit, a ringing is full-wave rectified by the rectifier and then passed through the threshold circuit to detect voltages higher than a predetermined threshold level; the ringing is detected by measuring the period of those particular voltages.
Generally, a subscriber's line is apt to subject a ringing to various kinds of distortions depending upon various conditions of the line. In this situation, the problem with the above-described prior art ringing detection circuit is that both the positive and the negative polarities derived from full-wave rectification are applied to a single threshold circuit. Specifically, should a distortion greater than the threshold value be contained in the ringing, the output of the threshold circuit would fluctuate every time the signal varies over the threshold value, introducing errors in the period measurement. Stated another way, relatively high frequencies would be superposed as noise on the ringing and be measured to prevent the period of the ringing from being accurately measured. In this condition, an incoming call sometimes fails to be accurately detected despite that is has been adequately received.
To eliminate such measurement errors, an implementation has been proposed in which an integration circuit including a capacity is provided at the output of a photocoupler in such a manner as to remove a certain degree of noise component. This implementation is not fully satisfactory, however, because it is incapable of removing distortions having relatively long periods.